The present invention relates to a fuel cell separator, a process for production thereof, and a polymer electrolyte fuel cell. More particularly, the present invention relates to a fuel cell separator having high elasticity, high electrical conductivity, and good moldability and a process for production thereof. The present invention relates also to a fuel cell having good gas sealing performance and good impact resistance in which all or part of the separators are those which are mentioned above. The fuel cell is suitable for use as a mobile power source for cars, hybrid cars, and small ships.
A polymer electrolyte fuel cell is composed of tens to hundreds of fuel cells (as unit cells) connected together. Each fuel cell consists of two fuel cell separators 1 and one polymer electrolyte membrane 2 and two gas diffusion electrodes 3 which are held between the separators, each separator having a plurality of ribs 1a on both sides thereof, as shown in FIG. 1.
The above-mentioned fuel cell separator 1 is a thin platy body having a plurality of ribs la on both sides thereof and a plurality of gas feed grooves 4 on one side or both sides thereof, as shown in FIGS. 2A and 2B. The ribs 1a of the separator and the electrode 3 form passages 5 for fuel gas such as hydrogen and oxygen to be supplied and discharged. Therefore, the fuel cell separators are required to have high elasticity and good dimensional accuracy. Moreover, the fuel cell separators and unit fuel cells are required to have good gas seal performance to prevent the leakage of fuel gas, good resistance to cracking by tightening at the time of assembling, and good impact resistance for the fuel cell to be used as a mobile power source for automobiles.
To meet these requirements, there has been proposed a separator for polymer electrolyte fuel cell in Japanese Patent Laid-open No. Hei 11-297337. This separator is obtained by curing from a homogenous mixture composed of 100 parts by weight of carbonaceous powder and 10 to 100 parts by weight of thermosetting resin. According to this disclosure, the carbonaceous powder is a graphite powder having a maximum particle diameter of 125 μm or below. There has also been disclosed a fuel cell separator in Japanese Patent Publication No. 2000-100453. This separator contains expansible graphite particles having a number-average particle diameter of 25 μm or above, preferably 25 to 500 μm.
Conventional fuel cell separators are made of a highly filled material which encounters difficulties in injection molding. The disadvantage of the above-mentioned separators containing graphite powder is that the fine graphite powder lowers the fluidity of the molding material, adversely affecting injection moldability and mechanical properties. Flake graphite has such a low bulk density that the material containing it slips on the screw of the injection molding machine, making itself incapable of molding. Graphite with a comparatively high bulk density is easily broken into fine powder at the time of mixing or injection molding.
Since a unit cell merely produces a low voltage, it is necessary to connect tens to hundreds of unit cells if a practical output (up to hundreds of kW) is to be obtained. Therefore, there is an urgent demand for a technology that permits efficient mass production of fuel cell separators having a uniform shape free of strain and thickness variation.
Conventional fuel cell separators are made of a composition composed of a thermosetting resin such as phenolic resin, and graphite. This composition is incorporated with a large amount of graphite so that the resulting separator has electrically conductivity as required. Therefore, it lacks fluidity and presents difficulties in injection molding. Actual production of separators is by compression molding, which involves heating at 150 to 160° C. and pressing at 14.7 to 29.4 MPa for 5 to 10 minutes. Compression molding is slow and inefficient for mass production.